Rooftop drag reducing devices have come into widespread use on trucks, particularly on trucks having fifth wheels pulling semi-trailers. These devices direct the air passing over the top of a truck in such a way that the wind hitting the flat front of the trailer being pulled is minimized. A major practical problem with these devices has been that the majority of them in use are molded in one piece, usually of fiber glass reinforced plastic. The devices cannot be mounted to the trucks at the factory, as the trucks are shipped multiply with the second and third truck, for example, having their front wheels mounted to the fifth wheel of the preceding truck. This raises the drag reducing devices on the second and third trucks and following trucks to heights that are greater than low bridges that the trucks have to go under during shipment. For this reason the majority of drag reducing devices are shipped to the truck dealer, usually unpainted. They are painted by the truck dealer to match the truck on which they are to be mounted and then mounted to the truck by the truck dealer. Since this is done at the truck dealer' s job shop, it is a more expensive procedure than would be encountered in painting and mounting the devices at the factory. An additional problem encountered is that some truck dealers substitute drag reducing devices not approved by the factory, which may not provide the drag reduction of the factory devices and may be subject to structural failure during use. The factory approved devices are highly engineered and tested to provide the maximum possible drag reduction for the particular truck model for which they are designed and are subjected to life tests and designed against structural failure for the lifetime of the truck. Another problem encountered with prior art one-piece devices is that they are too high for conventional towing when the truck breaks down.
Certain prior art rooftop drag reducing devices have been provided with collapsible features in an attempt to overcome these problems. However, they have been relatively expensive and subject to other forms of failure. For example, inflatable devices have been proposed, but these have not gained widespread acceptance because of their increased cost; the additional equipment required to inflate and deflate them; and, their being subject to tears and leaks. Other devices have been proposed but have not come into widespread use such as a two piece device where the top may be brought down with its sides outside the lower portion of the device. Such devices have the disadvantage of requiring supplementary support struts (or sway braces) to resist side loads imposed by wind buffeting and cab movement. This has been brought about by the fact that previous designs have been comprised of an inclined front-top, and two sides, but no rear surface to resist sway.
Generally, prior art devices have had top portions which either overlap bottom portions or folding plates or bellows. In all cases these designs have required various types of sway-bracing as well as some type of lifting and support hardware to keep the upper portion in position for its operating mode. These requirements have added substantial cost, weight, and complexity, and engendered reliability problems in the aerodynamic devices. The increased weight requires increased structure in the cab roof and supporting portions of the truck. This greatly increases the weight of the truck and thus reduces fuel economy.
In addition to the parts required to resist the various loads imposed on the device, the device itself has generally required a number of individual molded and fabricated components: top portions, side portions, folding plates, bellows, etc. It should be borne in mind that ideally a different device must be provided, matched to each truck model of each manufacturer. Multi-part devices at least double the manufacturer's mold cost.
In some cases such designs have had to be removed from the market because of structural failures; others have been discarded in the development and prototype phase as being impractical.